So have you ever checked your HTML and/or CSS through a validator? I checked mine and have about 570 errors and 8 warnings. Many of them look repetitive. What would it cost to have it all "cleaned up"? And will this effect my site? My site seems just fine right now. What has your experience been?
It's not really that hard to correct it. Somtimes theres errors that can cause multiple problems. I would suggest using the HTML tidy clean up option and use it to fix your code. That usually helps me, although my errors usually come from laziness like not putting in alt tags or something.
If you are familiar with HTML code yourself, then it definitely shouldn't be too dificult to clean up the website yourself. Once you start looking into the report that the HTML Validator gives you, you'll find tons of trivial errors (missing alt attributes, duplicate ids on various elements, etc.) that can be fixed in no time. The most problematic thing in my experience is when you have some JavaScript code to manipulate the front-end of your website - in this case you might need to fix quite many things in order for everything to work correctly and be "Valid". For instance, you should replace all duplicate ids with a certain class and change the jQuery selectors (for instance) from $("div#someID") into $("div.theNewClass") and so on. However, even though all my websites are HTML valid, it doesn't really make that much difference as long as your website is displayed properly in all major browsers. I hope this helps you a bit.
best way to tackle it (if your errors aren't being caused by JS as Aaron mentioned above) is to start with the first error and work your way down. Fix an error, revalidate-- because as already pointed out, errors cascade. Fixing one can remove dozens. Though with as many errors as you posted, it's likely overall bad coding (or possibly a bad doctype) setting most of those off. Those are harder to fix. It's easier to write something in good English to begin with, than to try to fix really really bad English.
It depends on the source of the errors. If most of them are simply deprecated tags, it really shouldn't be much more than a batch solution. On the other hand, if you're looking at multiple missing alt tags and the like, it may become a bit more time consuming. If you want to send me the URL of your site, I'd be glad to shoot you an estimate of the "repairs", but I don't think it'd be much work.
I'd almost say it the other way around. Missing alt attributes, if the images still have nothing to say, could be batch-added as alt="" to any and all HTML images (afterwards, of course, appropriate next should be added to those who actually need the text). Same could be said for closing slashes on empty elements. But for every <center> tag, you've got to set a width and automargins on either a replacement element, or the nearest block child of that <center> tag (which might mean adding an id as well). An <embed> likely needs a whole new <object> rewrite, even if it's an <embed> inside an <object>. Every <font> tag would need some styling marker (like a class) needed to set those text styles in the CSS (unless it's the same single style and the tag was just thrown everywhere). <applet>s would need to... I dunno what applet even did, so, dunno what it would become. iframes... might require a whole backend solution, depending on what they were doing.
I'm designer and HTML/CSS coder and I always try to do the most compatible and valid wbesites as I can.
Erm . . . Yes. That's what you're meant to do. Ouch! You should have validated repeatedly throughout the process of constructing the site. That way, you would have caught the errors early. Do it yourself for free. It should be very straightforward. If you get stuck, just ask for help. Your site is not fine. It has 570 errors and 8 warnings. Once you clean up the errors, then it'll be fine.
This one - http://validator.w3.org/ Or similar to it through a Firefox addon. Either way W3 is the one who is trying to uniform it all.